How to make clothing and body move as one?
In my opinion you should :

Apply the rotation of the armature (CtrlA &gt; Apply Rotation)
Get rid of the part of the mesh you don’t need, for example the arms, as you have sleeves, but keep everything if needed (but you’ll have to do more corrections after parenting), and anyway keep a unchanged version somewhere.
Join all the objects together with CtrlJ
Unparent the object with AltP &gt; Clear and Keep Transformation
Parent With Automatic Weight, make the needed corrections in Weight Paint mode or in the Properties panel &gt; Object Data &gt; Vertex Groups list and with the Assign button